The invention relates to an adsorptive separation process used to prepare a feed stream for a steam cracking process unit. The invention more specifically relates to an adsorptive process used to produce a high purity normal paraffin stream used as a feed stream to a steam cracking process.
Steam cracking, which is the thermal cracking of hydrocarbons in the presence of steam, is used commercially in large scale industrial units to produce ethylene and to a lesser extent propylene. These pyrolysis units are often charged a naphtha boiling range feed stream. The typical petroleum derived naphtha contains a wide variety of different hydrocarbon types including normal paraffins, branched paraffins, olefins, naphthenes, benzene, and alkyl aromatics. It is known in the art that paraffins are the most easily cracked and provide the highest yield of ethylene and that some compounds such as benzene are relatively refractory to the typical cracking conditions. It is also known that cracking normal paraffins results in a higher product yield than cracking iso-paraffins. A paper entitled Separation of Normal Paraffins from Isoparaffins presented by I. A. Reddock, et al, at the Eleventh Australian Conference on Chemical Engineering, Brisbane, Sep. 4-7, 1983 discloses that the ethylene yield of a cracking unit can be increased if it is charged a C5-C9 stream of normal paraffins rather than a typical C5-C9 natural gasoline.
The separation of the myriad components of a petroleum naphtha into specific structural types by fractional distillation, a form of fractionation, is prohibitively expensive and complicated and any attempt to improve the character of the naphtha as a steam cracking feed must therefore employ other means which act on a class of structural types, such as extraction.
The benefits of separating the various classes of hydrocarbons in petroleum fractions have led to the development of a number of different techniques which separate the hydrocarbons by type rather than individual molecular weight or volatility. For instance, various forms of liquid extraction can be used to remove aromatic hydrocarbons from a mixture of aromatic and paraffinic hydrocarbons. Adsorptive separation techniques have been developed to separate olefins from paraffins and to separate normal (straight chain) paraffins from non-normal, e.g. branch chain paraffins and aromatics. An example of such a process is described in U.K. patent Application 2,119,398 which employs a 5A zeolite having crystals larger than 5 Angstroms to selectively adsorb straight chain hydrocarbons to the exclusion of non-straight chain hydrocarbons and sulfur compounds.
There are great economic benefits to a large scale unit if an adsorptive separation is performed in a continuous manner, and methods to do this have been developed. U.S. Pat. No. 4,006,197 to H. J. Bieser and U.S. Pat. No. 4,455,444 to S. Kulprathipanja et al describe techniques for performing a continuous simulated moving bed (SMB) adsorptive separation process for the recovery of normal paraffins, which is the preferred mode of operating the adsorptive separation zone of the subject invention. The Bieser reference describes the fractionation of the raffinate and extract streams to recover desorbent which is reused in the process.
U.S. Pat. No. 3,291,726 issued to D. B. Broughton also describes the use of simulated moving bed technology to separate normal paraffins from a petroleum derived fraction. This reference further describes that a suitable desorbent for use in the process may be provided by fractional distillation of the unit feedstock and the raffinate and extract removed from the adsorption zone.
The invention is an adsorptive separation process which reduces the cost of separating normal paraffins from a broad boiling point range naphtha hydrocarbon fraction. The invention thereby provides an improved method for recovering a broad boiling mixture of normal paraffins which is highly suitable as a feed to a steam cracking unit intended to produce ethylene. It simultaneously produces a very desirable catalytic reforming feed stock. Overall cost reduction and process simplification are obtained in part by using selective adsorption to recover normal paraffins, with the desorbent used in the adsorption zone being derived from the naphtha feed stream of the overall process. This reduces the need to recover the desorbent for recycling.
A broad embodiment of the invention may be characterized as a process for preparing a feedstream to be charged to a steam cracking unit, which process comprises passing a process feed stream comprising C5 through C9 hydrocarbons including C5 through C9 normal paraffins into a first fractionation zone, and separating the hydrocarbons entering the first fractionation zone into a first process stream rich in C5 paraffins and a second process stream comprising C6 through C9 hydrocarbons; passing the second process stream into an adsorption zone of an adsorptive separation zone and selectively retaining normal paraffins on an adsorbent located in the adsorption zone to yield a raffinate stream comprising non-normal C6 through C9 hydrocarbons; passing the first process stream into a desorption zone in the adsorptive separation zone as at least part of a desorbent stream and removing normal paraffins from adsorbent present in the desorption zone to yield an extract stream comprising C6 through C9 normal paraffins and C5 paraffins; separating at least a portion of the extract stream in a second fractionation zone into a third process stream comprising C5 paraffins and a fourth process stream comprising C6 through C9 normal paraffins; and passing the fourth process stream into a cracking zone producing ethylene.